Optical thin film and mirror using the same

ABSTRACT

To provide an optical thin film structure capable of efficiently dissipating heat in an optical thin film which is generated upon irradiating a surface of an X-ray mirror made up of the optical thin film with an X-ray. The optical thin film having an isotope purity higher than a natural isotope abundance ratio is formed on a mirror to increase heat conductivity of the optical thin film itself and quickly dissipate heat accumulated in the thin film to the outside of an optical system. Consequently, the mirror having high reflectively can be obtained, in which a fine structure of the optical thin film is by no means broken.

TECHNICAL FIELD

The present invention relates to an optical thin film and a mirror usingthe same. In particular, the invention relates to a multilayer filmmirror used in a soft X-ray region.

BACKGROUND ART

In a soft X-ray region, light is significantly absorbed in allsubstances and in addition, a refractive index approximates 1. Hence, itis impossible in principle to make use of a lens operation due to itsrefraction. To that end, an optical system is made up of a mirror. Insuch a case, any general reflector made of a single-layer film has adirect incidence reflectivity of almost 0 and thus is far fromfunctioning in the optical system. In contrast, a multilayer film madeof a material relatively low in light absorption can function as adirect incidence mirror optical element, making it possible to use areflective optical system realized by the utilization of effectsthereof.

Heretofore, a reflector has been developed in order to obtain theaforementioned optical element such as a mirror, the reflector beingmade of a multilayer film including ultra-thin films formed byalternately stacking two kinds of substances, that is, A and B, up to atleast several ten layers and defining a number of reflective surfaces asinterfaces therebetween, with a thickness of the wavelength order thatis determined based on an optical interference theory that phases ofreflection waves from the respective interfaces match with each other.In order to obtain a high reflectivity, it is necessary to choose aproper combination of two substances, that is, the two substances A andB each having as small absorption coefficient as possible and havingreflective indexes n_(A) and n_(B), respectively with a large differencetherebetween. As a substance pair that enables the highest reflectivitywithin an incident wavelength range of 11 nm to 14 nm in a soft X-rayregion, an alternate multilayer film made of Mo and Si is exemplified(see Japanese Patent No. 3101695, for example). The multilayer film isformed by a thin film formation technique such as magnetron sputtering,EB evaporation, or ion beam sputtering.

When the multilayer film made of Mo and Si is formed with the abovetechnique, a central portion and peripheral portion of a film that isbeing formed are different in temperature, leading to a temperaturevariation. This causes a difference in how diffusion proceeds in themultilayer film, leading to an uneven film.

Further, a soft X-ray mirror made up of the multilayer film onlyreflects an X-ray having such a wavelength as to meet a reflectioncondition from among incident soft-X rays and absorbs almost all theX-rays having other wavelengths than the above wavelength. In short, ifan intense soft X-ray enters the multilayer film mirror, an energy ofthe X-ray absorbed in the multilayer film heats the multilayer film. Thetemperature rise of the multilayer film is supposed to reach aboutseveral hundreds of 0° C. in the case of synchrotron radiation, althoughdepending on an intensity of a soft X-ray incident on a reflector or anabsorptivity for the X-ray of the multilayer film. Therefore, when themultilayer film is heated, its fine structure is broken and changed.

In general, when a considerably uneven multilayer film is used or thefine structure of the multilayer film is broken, the reflectivitythereof drops. To prevent such a situation that heat is locallyaccumulated in the multilayer film or its structure is broken, the heatshould be dissipated from the inside of the multilayer film through heattransfer. A soft X-ray mirror made up of such a multilayer film is setin a vacuum when used as a component of an exposure device. In a vacuum,the heat transfer due to gases does not occur unlike the use in the air.Hence, the heat of the heated multilayer film needs to be transferred toa substrate through the multilayer film.

To that end, there has been proposed a method of forming a multilayerfilm mirror that easily induces heat transfer. With conventionaltechniques, heat is dissipated from a multilayer film by providing acooling mechanism for cooling the multilayer film (see Japanese PatentApplication Laid-Open No. 05-119208, for example). Also, the followingmethod has been developed. That is, a substance of a high heatconductivity (heat transfer layer) is provided in contact with amultilayer film and heat is dissipated from the multilayer film (seeJapanese Patent Application Laid-Open No. 05-333199, for example).

However, in the aforementioned conventional methods of dissipating heatfrom the multilayer film, heat radiation property of the multilayer filmitself is not taken into account. Unless the heat conductivity of themultilayer film itself increases, the heat is not released from themultilayer film.

DISCLOSURE OF THE INVENTION

The present invention has an object to provide an optical thin filmwhose heat resistance is improved by increasing a heat conductivity of amultilayer film itself to dissipate and release heat in the multilayerfilm and an X-ray mirror using the same.

In order to attain the above-mentioned object, according to a firstaspect of the present invention, there is provided an optical thin filmwhich has a composition with an isotope purity higher than a naturalisotope abundance ratio.

According to a second aspect of the present invention, there is providedan optical thin film, including at least a pair of alternate layersdifferent in composition, in which at least one of the alternate layersis the optical thin film according to the first aspect of the invention.

According to a third aspect of the present invention, there is providedan optical element, including the optical thin film according to thefirst or second aspect of the invention.

According to a fourth aspect of the present invention, there is providedan X-ray mirror, including the optical thin film according to the secondaspect of the invention.

According to a fifth aspect of the present invention, there is providedan X-ray mirror, including the optical thin film according to the secondaspect of the invention, in which one of the alternate layers containsMo and the other layer contains Si.

According to the present invention, it is possible that the optical thinfilm has the composition with the isotope purity higher than the naturalisotope abundance ratio to thereby increase a heat conductivity of anoptical film and heat accumulated inside the optical film is transferredand released to the outside of the film to thereby enhance the heatresistance of the optical film.

Also, in a soft X-ray multilayer film mirror made up of the aboveoptical film, a fine structure of the multilayer film is not broken eventhrough X-ray irradiation and maintains a high reflectivity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view of a soft X-ray mirror according toan embodiment of the present invention; and

FIG. 2 is a schematic sectional view of a soft X-ray mirror according toanother embodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, specific description will be given of examples of thepresent invention with reference to the accompanying drawings.

(Example 1)

FIG. 1 is a schematic sectional view of a soft X-ray multilayer filmmirror according to the present invention.

A soft X-ray multilayer film mirror 9 is made up of a multilayer film 2obtained by alternately stacking an Si layer 3 and an Mo layer 4 on asubstrate 1 by magnetron sputtering. A holder 6 holding the multilayerfilm mirror 2 allows a cooling medium 5 such as water to flow therein.Also, the mirror has an O ring 7 that prevents the cooling medium 5 fromleaking to the outside of the holder 6. With this arrangement, it ispossible to cool the multilayer film 2 that is apt to increase itstemperature due to the incidence of a soft X-ray 8.

The substrate 1 is preferably made of a material of high heatconductivity, for example, Si, Ni, Cu, or Ag.

When the Si layer 3 was formed on a surface of the substrate 1,sputtering was performed using Si as a target with an isotope purityhigher than that in a natural state.

Similarly to the above, when the Mo layer 4 was formed, sputtering wasperformed using Mo as a target with an isotope purity higher than thatin a natural state.

In such a way, the Si layers 3 and the Mo layers 4 were alternatelyformed on the surface of the substrate 1.

Adopted as a method of obtaining a target having a high isotopic puritywas a method of extracting target substances on an element mass (isotopemass) basis through centrifugation.

The formed Si layer 3 contained ²⁸Si at an abundance of 99%. In thiscase, the heat conductivity of the Si layer 3 was about 0.32cal/sec.cm.deg. This value significantly exceeded a natural heatconductivity of Si, that is, 0.20 cal/sec. cm.deg.

The reason why the heat conductivity of the Si layer 3 increased isdiscussed below.

That is, in a natural state, three different isotopes of Si coexist withmass numbers of 28, 29, and 30, respectively. The way to transmitvibrations varies among the different isotopes. Therefore, the heatconductivity falls to hinder the temperature from decreasing. If atomswith the same mass number could be exclusively gathered, atomicvibrations could be more efficiently transmitted. The isotope purity of²⁸Si whose abundance in a natural state is 92.23% is further increasedto obtain ²⁸Si in a single crystal form with the abundance of 99.92%, sothat the heat conductivity is 60% higher than that of Si in a naturalstate.

In the case of Mo as well, its heat conductivity is enhanced byincreasing the purity of a specific isotope. A layer containing ⁹⁸ Mo atan abundance of 90% was used as the Mo layer 4.

When the multilayer film thus prepared is formed on the mirror surface,heat in the multilayer film is dissipated through the substrate. Withthis method, the heat resistance of the Mo/Si multilayer film isimproved to realize a soft X-ray multilayer film mirror with a highreflectivity, in which a fine structure of the multilayer film is by nomeans broken.

As a result of comparing film characteristics of any conventionalmultilayer film reflector and the multilayer film reflector according tothe present invention after the irradiation with the same amount of softX-rays for several hours to several tens of hours, the formerexperienced film breakage and its reflectivity decreased by 2 to 3%. Incontrast, the latter had excellent durability and was, therefore, freeof a reflectivity decrease.

(Example 2)

Next, a soft X-ray multilayer film mirror according to anotherembodiment of the present invention will be described.

The soft X-ray multilayer film mirror of this example was made up of themultilayer film 2 obtained by alternately stacking the Si layer 3 andthe Mo layer 4 on the substrate 1 having a high heat conductivity bymagnetron sputtering as in Example 1. The Mo layer 4 was a layercontaining Mo with a natural isotopic abundance ratio and a heatconductivity of 0.34 cal/sec.cm.deg. On the other hand, the Si layer 3contained ²⁸Si at an abundance of 99% or more with a heat conductivityof about 0.32 cal/sec.cm.deg. This value approximates the heatconductivity of the Mo layer. As discussed above, a substantiallyuniform heat conductivity was attained throughout the multilayer film,so that the heat inside the multilayer film was able to be easilytransferred to the substrate 1. Thus, the heat resistance of the Mo/Simultilayer film was improved to realize the soft X-ray multilayer filmmirror with a high reflectivity, in which a fine structure of themultilayer film was by no means broken.

(Example 3)

Next, description will be given of a soft X-ray multilayer film mirroraccording to still another embodiment of the present invention,referring to FIG. 2.

The soft X-ray multilayer film mirror of this example was obtained byforming on the substrate 1 having a high heat conductivity, the Si layer3 and the Mo layer 4, which sandwiched a B₄C layer 10 that is anintermediate layer, by magnetron sputtering like Example 1. Themultilayer film 2 of a four-layer structure obtained by forming the Silayer 3, the B₄C layer 10, the Mo layer 4, and the B₄C layer 10 in thisorder was regarded as a multilayer film in one cycle. The multilayerfilm 2 was formed by stacking the film of one cycle. According to thismethod, the Si layer 3 and the Mo layer 4 were unadjacent in themultilayer film, making it possible to prevent the production ofinterface compounds that would naturally occur between the Si layer 3and the Mo layer 4.

An layer containing ²⁸Si at an abundance of 99% or more and a layercontaining ⁹⁸Mo at an abundance of 90% were used as the Si layer 3 andthe Mo layer 4, respectively. The heat conductivity thereof was enhancedby increasing the purity of a specific isotope. Further, in the B₄Clayer 10, the heat conductivity of the B₄C layer 10 was 50% higher thanthat of B₄C in a natural state by using B₄C containing ¹¹B at anabundance of 99% or more. Hence, the heat resistance of theMo/B₄C/Si/B₄C multilayer film 2 was improved to realize a soft X-raymultilayer film mirror with a high reflectivity, in which a finestructure of the multilayer film was by no means broken.

This application claims priority from Japanese Patent Application No.2003-315438 filed Sep. 8, 2003, which is hereby incorporated byreference herein.

1. An optical element comprising a substrate and at least a pair ofalternate layers provided on the substrate, the alternate layers eachhaving an Mo layer and an Si layer, wherein the Mo layer contains 90% ormore of ⁹⁸Mo and the Si layer contains 99% or more of ²³Si.
 2. Anoptical element comprising a substrate and at least a pair of multilayerfilms of a four-layer structure provided on the substrate, each of theat least the pair of the multilayer films having an Si/B₄C/Mo/B₄Cstructure in this order from the substrate, wherein the Mo layercontains 90% or more of ⁹⁸Mo, the Si layer contains 99% or more of ²³Siand the B₄C layer contains 99% or more of ¹¹B.
 3. An X-ray mirrorcomprising the optical element according to claim
 1. 4. An X-ray mirrorcomprising the optical element according to claim 2.